Mineral analysis systems, such as the Qemscan and MLA available from FEI Company, Hillsboro, Oreg., have been used for many years to analyze mineral samples. To determine the type and relative quantity of minerals present in a mine, a sample in the form of small granules, is fixed in epoxy in a mold and the mold is placed in a vacuum chamber. An electron beam is directed toward a sample and, in a process called “energy dispersive x-ray spectroscopy” or “EDS,” the energies of x-rays coming from the sample in response to the electron beam are measured and plotted in a histogram to form a spectrum. The measured spectrum can be compared to the known spectra of various elements to determine which elements and minerals are present. FIG. 1 shows a typical sample 100 having granules 102 embedded in epoxy matrix 104.
It takes considerable time to accumulate an x-ray spectrum. When an electron in the primary beam impacts the sample, the electron loses energy by a variety of mechanisms. One energy loss mechanism includes transferring the electron energy to an inner shell electron, which can be ejected from the atom as a result. An outer shell electron will then fall into the inner shell, and a characteristic x-ray may be emitted. The energy of the characteristic x-ray is determined by the difference in energies between the inner shell and the outer shell. Because the energies of the shells are characteristic of the element, the energy of the x-ray is also characteristic of the material from which it is emitted. When the number of x-rays at different energies is plotted on a graph, one obtains a characteristic spectrum, such as the spectrum of pyrite shown in FIG. 2. The peaks are named for the corresponding original and final shell of the electron that originated the x-ray. FIG. 2 shows the sulfur Kα peak, the iron Kα peak and the iron Kβ peaks.
Many primary electrons must strike the sample to produce sufficient x-rays to create an identifiable spectrum. Not every incoming electron will knock out an inner shell electron, and different inner shell electrons may be ejected, with the gap being filled by different outer shell electrons. Because the x-ray detector subtends a relatively small solid angle, only a relatively small number of the emitted x-rays are detected. The probability of an incoming electron causing the emission of a detectable x-ray of a particular energy depends on many factors, including the elemental composition of the sample, the energy of the incoming electron, the geometric relation between the electron beam, the sample surface, and the detector, the likelihood of a particular inner shell electron absorbing the energy of a primary beam electron, and the likelihood of a particular outer shell electron angle dropping to the vacancy in the inner shell.
Moreover, the energy measurement of the electron, like any measurement, has an inherent error. Thus, rather than a spectrum showing a peak corresponding to an electron transition at a single value, the peak will be spread over a range of values. Because the peaks from different transitions of different elements can overlap, a large number of x-rays is collected to more precisely define the locations of the peak. Several million x-rays, each referred to as a “quant,” are typically detected to form a reliable spectrum in which the most important peaks can be identified with sufficient confidence. U.S. Pat. Publication No. 2011/0144922, which is assigned to the assignee of the present application, describes an algorithm that allows elements to be determined with reasonable confidence using a smaller number of detected x-rays, for example, a thousand x-rays.
Other emissions besides characteristic x-rays are detectable when an electron beam impacts a sample surface. Background, or Bremsstrahlung, radiation comprise x-rays spread over a wide range of frequencies and can obscure characteristic x-ray peaks. Secondary electrons, Auger electrons, elastically and inelastically forward or backward scattered electrons, and light can be emitted from the surface upon impact of a primary electron beam and can be used to form an image of the surface or to determine other properties of the surface. Backscattered electrons are typically detected by a solid state detector in which each backscattered electron is amplified as it creates many electron-hole pairs in a semiconductor detector. The backscattered electron detector signal is used to form an image as the beam is scanned, with the brightness of each image point determined by the number of backscattered electrons detected at the corresponding point on the sample as the primary beam moves across the sample.
Backscattering of electrons depends on the atomic number of the elements in the surface and upon the geometric relationship between the surface, the primary beam, and the detector. The backscattered electron image therefore shows contour information, that is, boundaries between regions of different composition, and topographical information. Obtaining a backscattered electron image requires collecting only a sufficient number of electrons at each point to produce a reasonable contrast between points having different properties and so is much faster than obtaining a sufficient number of x-rays to compile a complete spectrum at each point. Also, the probability of an electron being backscattered is greater than the probability of the electron causing the emission of a characteristic x-ray of a particular frequency. Obtaining sufficient backscattered electron image data at a single dwell point typically takes less than a microsecond, whereas acquiring sufficient x-rays to obtain an analyzable spectrum at a single dwell point typically takes more than a millisecond.
In one mode of operating the MLA system, an image is first acquired using a backscattered electron detector, and the image is then processed to identify regions that appear from the contrast to have the same elemental composition. The beam is then positioned at the centroid of each identified region for a longer dwell time to collect an x-ray spectrum representative of the region. X-rays generated during the backscattered electron detector scan are not used.
While some systems include a “fast mapping” mode to produce a “spectrum cube,” that is, a two-dimensional map of a sample, with the composition of the material at each point providing the third dimension of the cube, the “fast map” still requires sufficient time at each dwell point to collect enough x-rays to determine the type and quantity of elements present at the pixel.